Thermoplastic polyester materials have many commercial applications and are commonly used in fabrics, films, and containers. Polyesters are popular due the their mechanical strength, flexural characteristics, clarity, thermal stability, impact strength, and permeability characteristics. In the beverage industry, in particular, poly(ethylene terephthalate) (PET) has emerged as a major material for bottling carbonated as well as non-carbonated beverages.
In spite of these positive characteristics, PET possesses some significant limitations. One of these limitations is related to its rate of crystallization. The rate of crystallization of PET is slow relative to some other plastics, such as poly(butylene terephthalate) (PBT) and high density polyethylene (IIDPE). A consequence of this relatively slow rate of crystallization is that relatively long cycle times are required to achieve crystallinity in PET, and when achieved, crystallinity is often accompanied by opacity, due to the relatively large size of crystallites formed by thennal crystallization. Crystallinity itself is often desirable in molded parts, due to the higher thermal and mechanical stability associated with crystallinity. Crystallinity is especially desirable when parts or containers will be subjected to elevated temperatures.
Prior art methods to increase the rate of crystallization of PET have been directed toward incorporation of inorganic compounds, polyolefins, and salts into PET to act as nucleation aids. The use of these inorganic compounds and salts, while having a positive effect on the crystallization rate, have other adverse consequences. In particular, inorganic compounds and salts typically lower the thermal stability of PET. Salts such as sodium chlorobenzoate react with the polyester, and adversely affect the molecular weight. In addition, their effectiveness is time dependent; that is, the effectiveness of a given amount of sodium chlorobenzoate is dependent on the processing time in the polymer melt. Polyolefin nucleation agents, while increasing the rate of crystallization of PET, are relatively ineffective (requiring high loading levels), inherently affect clarity, and are thermally unstable at normal polyester processing conditions.
Thus, there exists a need in the art for a method to accelerate the rate of crystallization of PET that will allow crystalline PET parts to be manufactured without incurring long cycle times. In particular, there exists a need for a method to accelerate the rate of crystallization of PET that does not suffer the limitations of prior art methods.